1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a copier, a facsimile machine, a multi functional machine including functions of the printer and an image reading device, etc., and in particular, to a fixing device provided in the image forming apparatus.
2. Discussion of the Background Art
An image forming apparatus employing an electro-photographic system generally includes a drum or belt type photoconductive member as an image bearer, a charge device, and an exposure device, each arranged around the photoconductive member. The image forming apparatus further includes a developing device, a cleaning device, and a charge-removing device, also disposed around the photoconductive member. Specifically, the surface of the photoconductive member is uniformly charged, and an exposure process is executed to form a latent image thereon corresponding to text or an image to be printed or the like. Then, the latent image is developed into a visible toner image using toner. The toner image is transferred onto a transfer sheet either directly or indirectly via an intermediate transfer belt. The toner image on the transfer sheet is then fixed on the transfer sheet when passing through the fixing device, thereby printing is completed.
The fixing device includes a fixing member, such as a fixing roller, a fixing sleeve, a fixing belt, etc., and a pressure member, such as a pressure roller, etc. The fixing member and the pressure member contact and press into each other thereby forming a nip therebetween, through the transfer sheet is bearing the toner image is conveyed. Thus, the toner image on the transfer sheet is heated, pressed, and fixed in place on the transfer sheet when passing through the nip. A vicinity of a surface of one of the fixing member and the pressure member typically includes an elastic layer. Thus, in a conventional fixing device, when small size transfer sheets are repeatedly or normal size transfer sheets are longitudinally conveyed, portions of the surface layer at edges of the transfer sheet become rough and thereby edge marks are created. Subsequently, when a larger size transfer sheet is fed in such a situation, the edge marks are transferred onto that large image.
To resolve such a problem, avoiding lines in the image or uneven glossiness, a fixing device of Japanese Patent Application Laid Open No. 2008-40364 (JP-2008-040364-A) employs a refreshing roller that rotatavely engages with surface of a fixing roller to make a large number of friction cuts in the surface of the fixing roller and thereby make the surface of the fixing roller uniform. Further, although it does not improve the surface roughness created by passage of the transfer sheet, JP-2006-154540-A employs a cleaning roller in a fixing device to contact and erase contact marks on the surface of a fixing belt created by a contact-type temperature probe contacting the fixing belt.
Both of the above-mentioned approaches employ another member that contacts the surface of the fixing member to either reduce the surface roughness or to remove steins or dirt thereon. However, with this technology, when transfer sheets of a small size are repeatedly conveyed or normal size transfer sheets are longitudinally conveyed, portions on the surface of a fixing member or a pressure member corresponding to edges of the transfer sheet are roughened and lines drawn thereat, creating lines in images on large size transfer sheets as a problem to be resolved.
The above-mentioned problem occurs in a nip created by the fixing member and the pressure member pressing into each other on their surfaces where edges of a transfer sheet pass through due to concentration of stress thereat, induced in proportion to a thickness of the transfer sheet. Consequently, when a fixing belt or sleeve having an elastic surface layer on a metal substrate is employed and a thick transfer sheet is conveyed through the nip, the substrate of the fixing belt or sleeve is likely to be either deformed or damaged due to the concentration of stress on the positions corresponding to edges of the transfer sheet, producing an abnormal image.